Robust 2D layered MXene matrix–boron carbide hybrid films for neutron radiation shielding

Large-scale fabrication of neutron-shielding films with flexible or complex shapes is challenging. Uniform and high boron carbide (B4C) filler loads with sufficient workability are needed to achieve good neutron-absorption capacity. Here, we show that a two-dimensional (2D) Ti3C2Tx MXene hybrid film with homogeneously distributed B4C particles exhibits high mechanical flexibility and anomalous neutron-shielding properties. Layered and solution-processable 2D Ti3C2Tx MXene flakes serve as an ideal robust and flexible matrix for high-content B4C fillers (60 wt.%). In addition, the preparation of a scalable neutron shielding MXene/B4C hybrid paint is demonstrated. This composite can be directly integrated with various large-scale surfaces (e.g., stainless steel, glass, and nylon). Because of their low thickness, simple and scalable preparation method, and an absorption capacity of 39.8% for neutrons emitted from a 241Am–9Be source, the 2D Ti3C2Tx MXene hybrid films are promising candidates for use in wearable and lightweight applications.

The major problem is the comparison of the macroscopic cross-section between the reported values in this work and values from various references, e.g.shown in Figure 4 and Table S4.Nearly all referenced values (Ref. 15,19,21,22,32,(45)(46)(47)(48)(49) were done without specifying the use of "isotope enriched 10-B or 10-B-compounds" in their experimental or theoretical works, and one would assume "non-isotope enriched B or B-compounds" to be the feasible/economical option for the groups in the references.In Ref. 44,two products are commercial (Boral™ and Metamic™) and hence difficult to assess if isotope-enriched raw materials are used or not.Since the natural abundance of 10-B isotope is about 20 at.%, while the author of this work is using 10-B powder with >95% isotope enrichment (Supplementary Note 2), the difference in neutron capture crosssection would be about 5 time without going into detail calculation and considering the scattering of other elements.Such difference is backed by a theoretical work done by Backis et al., EPJ Techniques and Instrumentation (2022) 9:8, that the difference of the macroscopic cross-section between 99% isotope enriched 10-B4C and non-isotope enriched B4C (referred to as "natB4C" in the article) is about one order (see Figure 3 in the article).
Since the macroscopic interaction cross-section is a multiplication between the microscopic interaction cross-section and the atomic density, the 10-B isotope enrichment in the raw material will have a direct, and nearly linear, impact on the final property.Hence, if one would take the reported values in this work, and multiply them by 20% to match the 10-B concentration with other works, and scale them to a similar weight percentage in the matrices (say 60 wt.%), they become usual values that one can expect from regular 10-B4C powder.Similarly, the half-value layer thickness is a function of the macroscopic cross-section and therefore is strongly influenced by the enrichment.That also means the data comparison currently presented in the work are misleading to the reader at the first glance, and therefore the article should not be published in current status.

Significance:
As assessed in previous section, the reported neutron capture cross-section is likely not a breakthrough and hence lands not much impact on the performance of neutron shielding.
The report of a new B4C-containing composite for neutron shielding purpose can be interesting for certain applications, especially with a low H-content matrix instead of conventional polymer-based ones to reduce the high neutron scattering cross-section.However, there is no comparing studies in the report showing the new MBP composite is outperforming any commercially available neutron shielding materials comprising B or B4C, either flexible (e.g.MirroBor or  or not (e.g.Boral or Metamic).It is then difficult to argue the technical advance of the new reported composite with respect to available ones, especially in the areas that the authors repeatedly claimed -"robust" and "flexible".The authors need comparison results to show that MBP composites are, for example, mechanically more resilient, or thermally more stable, or less prominent to neutron scattering…etc.

Data and Methodology:
The data are mostly well collected, presented, and referenced if applicable, except the aforementioned issues in Figure 4 and Table S4, where the values of samples with isotope enrichment or not are compared.The data shown is convincing that the authors have produced the reported materials (Ti3AlC2 MAX phase, Ti3C2Tx MXene, MXene/PVA solutions, MBP solutions, and MBP coatings/films).It has even shown that the authors have studied and attempted to optimize the solutions systems for the best mechanical properties.
An issue to be fixed is the lack of information for the thickness measurements in Figure 3c.It is either not clearly, or not even, written how the thickness were measured from the painted layer of the area.In terms of large area shielding, it is also more interesting to see the uniformity (homogeneity) of the coating in terms of thickness and composition over the coated area, rather than an averaged thickness.

Analytical Approach:
There is no obvious issue of statistics that would influence the publication in current version of manuscript.
Suggested improvements: 1. Fix the data presentation in Figure 4 and Table S4, so there is no misleading information.Make a fair comparison study with literature values of the macroscopic neutron capture cross-sectionthe authors should look for values done with isotope enriched materials.Otherwise, the authors should present new results done with non-isotope enriched B4C for the comparison, and/or perform a detail theoretical study of their material (Ti3C2Tx/B4C/PVA).A scientific explanation should be provided if the composite has a higher cross-section than another B4C-based material with a similar weight percentage, instead of just reporting the values.2. Provide more results for the other properties of the new composite material, e.g.relevant mechanical properties or thermal stabilities, in comparison with existing materials.Especially the ones with very similar characteristics (flexible B4C-containing neutron shielding).As for now, it is not clear what the new composite material can provide to the field of neutron shielding since there are already materials reasonably flexible and robust.3. Provide mass density or atomic density of the MBP composites or films so it is easier to perform a calculation of the theoretical neutron interaction cross-section -not just for capture, but also for the overall interaction like fission and scattering.All interactions are important for the shielding, though the latter are often neglected due to lower values.Provide physical properties of the material will also help successors to assure the quality of their solutions, if one would like to reproduce.4.More detail regarding the thickness measurements in Figure 3c should be given in a clear way, including how it was done (SEM?AFM?) and how the area of interest was selected.It is the best, but not necessary, to provide a position-thickness relation to show the quality of thickness homogeneity.

Clarity and context:
The texts are in general well-written and the detail of the experiment is provided.The article is easy to follow and structured in a logical fashion.

Response to Reviewers' comments:
We would like to thank the Reviewers for their insightful comments and valuable suggestions on our manuscript entitled "Neutron radiation shielding with a robust 2D layered MXene matrix for boron carbide fillers".We have made careful revision of our manuscript based on comments suggested by the Reviewers.For ease of tracking, we are assigning numbers the Reviewers' comments as shown below.Comments are reproduced verbatim in italics, followed by our responses.

Key Results:
The authors made a report on synthesis of Ti3AlC2 MAX phase and chemically exfoliated Ti3C2Tx MXene, and subsequently prepared hybrid solutions with the MXene flakes, size-selected isotope-enriched 10B4C powder, and binder/solvent PVA in different concentrations.The MXene/B4C/PVA solutions were thereafter vacuum-filtrated or bladecoated into MBP composite films, as referred to by the authors.The authors showed and claimed the MBP films were mechanically flexible and moderately robust, and the B4C powders were homogeneously distributed within the MXene matrix.The most important claim was that the neutron shielding results showed MBP films' macroscopic (neutron capture) cross-section is one of the best values reported for synthetic B4C composites to date.

Validity: The authors provided detail synthesis and characterization information of the Ti3AlC2
MAX phase, the Ti3C2Tx MXene, and the MXene/PVA solution so one can be convinced with their results related to MAX/MXene.However, all of above steps have been reported before (as the references given by the authors), which justifies the decision of putting the synthesis and characterization in the supplementary information.Hence, the focus of the review is put mainly on the quality of the MBP solutions/films along with their performance in neutron radiation shielding.
The major problem is the comparison of the macroscopic cross-section between the reported values in this work and values from various references, e.g., shown in Figure 4 and Table S4.Nearly all referenced values (Ref. 15,19,21,22,32,[45][46][47][48][49] were done without specifying the use of "isotope enriched 10-B or 10-B-compounds" in their experimental or theoretical works, and one would assume "non-isotope enriched B or B-compounds" to be the feasible/economical option for the groups in the references.In Ref. 44, two products are   commercial (Boral TM and Metamic TM ) and hence difficult to assess if isotope-enriched raw materials are used or not.Since the natural abundance of 10-B isotope is about 20 at.%, while the author of this work is using 10-B powder with >95% isotope enrichment (Supplementary Note 2), the difference in neutron capture cross-section would be about 5 time without going into detail calculation and considering the scattering of other elements.Such difference is backed by a theoretical work done by Backis et al., EPJ Techniques and Instrumentation (2022) 9:8, that the difference of the macroscopic cross-section between 99% isotope enriched 10-B4C and non-isotope enriched B4C (referred to as "natB4C" in the article) is about one order (see Figure 3 in the article).
Since the macroscopic interaction cross-section is a multiplication between the macroscopic interaction cross-section and the atomic density, the 10-B isotope enrichment in the raw material will have a direct, and nearly linear, impact on the final property.Hence, if one would take the reported values in this work, and multiply them by 20% to match the 10-B concentration with other works, and scale them to a similar weight percentage in the matrices (say 60 wt.%), they become usual values that one can expect from regular 10-B4C powder.
Similarly, the half-value layer thickness is a function of the macroscopic cross-section and therefore is strongly influenced by the enrichment.That also means the data comparison currently presented in the work are misleading to the reader at the first glance, and therefore the article should not be published in current status.
Significance: As assessed in previous section, the reported neutron capture cross-section is likely not a breakthrough and hence lands not much impact on the performance of neutron shielding.
The report of a new B4C-containing composite for neutron shielding purpose can be interesting for certain applications, especially with a low H-content matrix instead of conventional polymer-based ones to reduce the high neutron scattering cross-section.However, there is no comparing studies in the report showing the new MBP composite is outperforming any commercially available neutron shielding materials comprising B or B4C,either flexible (e.g., or not (e.g., Boral or Metamic).It is then difficult to argue the technical advance of the new reported composite with respect to available ones, especially in the areas that the authors repeatedly claimed-"robust" and "flexible".The authors need comparison results to show that MBP composites are, for example, mechanically more resilient, or thermally more stable, or less prominent to neutron scattering…etc.

Reply on Validity and Significance:
We acknowledge that the reported neutron capture crosssection values, facilitated by isotope-enriched 10 B, may not be considered a breakthrough.However, our focus was on resolving the inherent challenges of integrating a high concentration of B4C within an ultrathin layer.Maintaining structural stability and uniformity under such conditions poses considerable difficulties.Commercially available neutron shielding materials typically possess millimeter-scale thickness and low B4C loading, as indicated by product specifications.For instance, Mirrobor TM has a mass content of over 83% at 2 or 5 mm, while SWX-238 Flex Boron contains more than 27.6 wt.% at 3.2 mm.We precisely controlled the nanostructures, particularly the size of MXene flake and B4C particles, and the interaction between the matrix and filler materials to ensure structural integrity.Additionally, the film was obtained using straightforward vacuum filtration or painting processes, eliminating the need for added pressure.This approach enabled us to fabricate films that span several hundred square centimeters, demonstrating a considerable coverage area.The simplicity and scalability of these fabrication techniques hold practical advantages, facilitating the production of films with desired thickness and coverage and spanning large areas without requiring intricate equipment or processes.

Data and Methodology:
The data are mostly well collected, presented, and referenced if applicable, except the aforementioned issues in Figure 4 and Table S4, where the values of samples with isotope enrichment or not are compared.The data shown is convincing that the authors have produced the reported materials (Ti3AlC2 MAX phase, Ti3C2Tx MXene, MXene/PVA solutions, MBP solutions, and MBP coatings/films).It has even shown that the authors have studied and attempted to optimize the solutions systems for the best mechanical properties.
An issue to be fixed is the lack of information for the thickness measurements in Figure 3c.
It is either not clearly, or not even, written how the thickness were measured from the painted layer of the area.In terms of large area shielding, it is also more interesting to see the uniformity (homogeneity) of the coating in terms of thickness and composition over the coated area, rather than an averaged thickness.

Reply on Data and Methodology:
Given the limited number of experimental samples, we recognize the limitations in comparing values with and without isotope enrichment.During our initial investigation into B4C hybridization with our synthesized 2D MXene, we encountered difficulties creating structurally stable composites due to morphological and surface characteristic variations.We fabricated the composite using a singular B4C material procured from an international supplier, following an optimization process to achieve optimal mechanical properties.We acknowledge the significance of conducting a comprehensive comparison with multiple B4C materials.To address this, we have included validated simulations that supplement the experimental data.The revised manuscript describes the methodology and simulation procedures to ensure transparency and foster reproducibility.By incorporating simulations, we aim to thoroughly understand the factors influencing the observed values in both cases and bolster the precision of our findings.
In the revised manuscript, we have provided detailed information regarding the method used to measure the thickness of the painted layer, ensuring clarity and accuracy in our methodology.Furthermore, we have included SEM images of the coated samples at multiple magnifications to underscore the importance of coating uniformity across a large area.These images demonstrate the homogeneity of the coating, visually affirming the uniformity across the coated area.
Overall, these revisions improve the clarity and comprehensiveness of our data and methodology, allowing for a more accurate and thorough evaluation of our results.

Analytical Approach:
There is no obvious issue of statistics that would influence the publication in current version of manuscript.

Suggested Improvements:
1. Fix the data presentation in Figure 4 and Table S4, so there is no misleading information.

Make a fair comparison study with literature values of the macroscopic neutron capture crosssection -the authors should look for values done with isotope enriched materials. Otherwise, the authors should present new results done with non-isotope enriched B4C for the comparison,
and/or perform a detail theoretical study of their material (Ti3C2Tx/B4C/PVA).A scientific explanation should be provided if the composite has a higher cross-section than another B4Cbased material with a similar weight percentage, instead of just reporting the values.

Reply on Comment (1):
To address this comment, we conducted Monte Carlo N-Particle (MCNP) simulations using non-isotope enriched B4C.The simulation results allowed us to perform a comprehensive and unbiased comparison with other B4C-based materials reported in the literature.We obtained data for macroscopic cross-sections with varying isotopic compositions and presented a detailed analysis and comparison in our revised manuscript.S4.N-MBP consistently exhibited a decrease in the macroscopic cross-section at all B4C contents (i.e., 20, 40, and 60 wt%), compared with the experimental results.This reduction is attributable to the inherently low neutron absorption of 11 B, which is the predominant isotope in natural B4C, as confirmed in a previous theoretical study. 52The fitted neutron absorbance with the increasing thickness of N-MBP demonstrated comparable or slightly higher capacities compared with those of conventional composites, thereby suggesting the utilization of MBP with natural B4C.In addition, E-MBP, which was simulated using the same B4C ratio as the actual MBP, exhibited a trend similar to that of the experimental results.This suggests structural uniformity and good agreement between the MCNP simulation and experimental findings.We would like to highlight that one of the key advantages of our MBP composite films is their extremely thin thickness compared with materials with millimeter-scale thickness, as indicated by the MSDS files provided for Mirrobor TM and SWX-238 Flex Boron.This thickness was achievable owing to the 2D nature of the MXene used as the matrix, which enabled a highly aligned structure.For mechanical properties, we have conducted additional tests, specifically tensile test of commercially available neutron-shielding material (Mirrotron, Mirrobor TM ), to evaluate the mechanical properties of our MPB film.The results exhibit higher mechanical strength and resilience compared to the existing materials, promising superior mechanical stability.In addition, thermogravimetric analysis results showed that the MBP film exhibited high thermal stability.i.e., it successfully withstood temperatures of up to 600 °C without structural decomposition.This indicates its potential for achieving excellent performance at elevated temperatures.
In our revised manuscript, we have made the necessary corrections and included additional results to enhance the understanding of the advantages offered by our MBP composite material.

*Revised manuscript
(page 5, line 22) When compared to a commercially available resin-based shielding material (Mirrotron, Mirrobor TM ), our MBP film showcased lower thickness and higher mechanical strength, suggesting a reduced mechanical degradation rate as a coating material (Supplementary Figure S15).
(page 9, line 35) The freestanding MBP hybrid films, with a density of ~2.0 g/cm 3 within the B4C weight fraction range of 20-60 wt.% (as shown in the calculations in Supplementary Table S6), can settle on the tips of a dandelion (Supplementary Figure S22a).Results of the thermogravimetric analysis (TGA) demonstrated that the hybrid films exhibited thermal stability below ~180 °C, with minimal weight loss below 5%, except for the thermal decomposition of the PVA binder (Supplementary Figure S22b).

Provide mass density or atomic density of the MBP composites or films so it is easier to
perform a calculation of the theoretical neutron interaction cross-section -not just for capture, but also for the overall interaction like fission and scattering.All interactions are important for the shielding, though the latter are often neglected due to lower values.Provide physical properties of the material will also help successors to assure the quality of their solutions, if one would like to reproduce.

Reply on Comment (3):
In the revised manuscript, we provided the weight percentages of each element (Ti, B, C, H, and O) in the MBP composite, based on assuming the presence of only Ti3C2 blocks, which is relevant to the MCNP thermal neutron absorption simulation (Supplementary Table S4).In addition, we included the mass density of the MBP composites by providing a detailed procedure for calculating the mass density using the measured weights and cross-sectional SEM measurements of the sample thickness.The calculated mass densities are presented in Supplementary Table S6.

*Revised manuscript
(page 9, line 19) These composites were characterized by distinct weight fractions of the 10 B isotope compared to the 11 B isotope as calculated in Supplementary Table S4.
(page 9, line 35) The freestanding MBP hybrid films, with a density of ~2.0 g/cm 3 within the B4C weight fraction range of 20-60 wt.% (as shown in the calculations in Supplementary Table S6), can settle on the tips of a dandelion (Supplementary Figure S22a).
(page 12, line 24) The weight percentage of each element (Ti, B, C, H, and O) in the MBP composite was calculated by assuming the presence of only one Ti3C2 block.The mass densities of the MBP composites were calculated using the following procedure for the MCNP simulations: All MBP samples were cut into 20 mm × 20 mm rectangles, and the actual weight of each sample was measured.To calculate the volume, the thicknesses of the samples were characterized using cross-sectional SEM images.Finally, the mass densities of the MBP composites were determined by dividing the weight by the volume.

Reply on Comment (4):
We appreciate your suggestion to provide more detail regarding the thickness measurements.Thickness measurements were performed using a digital blade micrometer.To accurately measure the thickness of the painted films, we subtracted the thickness of the stainless-steel foil from the total measured thickness.To investigate the thickness across the entire sample area, we randomly selected 20 different points and measured the thickness at each point.We have revised the manuscript to include this information and to clarify the methodology used.

*Revised manuscript
(page 20, line 5) The thickness was randomly measured at 20 different points on each sample using a digital blade micrometer.

Clarity and context:
The texts are in general well-written and the detail of the experiment is provided.The article is easy to follow and structured in a logical fashion.

References:
The authors are suggested to either replace the references in Figure 4 and Table S4 with other works done with isotope enriched 10-B, or redo the sample to match the level of enrichment with current references.Including some theoretical works showing the limits of the macroscopic cross-section of B4C and B4C composites will also help the readers to assess the performance of the materials.

Reply:
We have carefully considered your recommendations and thus revised the manuscript considerably.To ensure a more accurate and comprehensive comparison with previous studies, we extended our simulations to include different types of B4C powders with varying weight fractions of the 10 B isotope compared with the 11 B isotope.We denote these simulation scenarios as E-MBP and N-MBP, which represent the use of 10 B isotope-enriched B4C with a weight fraction of 32.4 and natural B4C with a weight fraction of 0.24, respectively.This allowed us to account for isotopic variations and performed unbiased comparisons with references.
We observed a consistent decrease in the macroscopic cross section of N-MBP at different B4C contents, which was not indicated in the experimental results.This reduction is attributed to the inherently low neutron absorption of 11 B, which is the predominant isotope in natural B4C.Subsequently, we analyzed the fitted neutron absorbance of N-MBP with different thicknesses and discovered comparable or slightly higher capacities compared with those of conventional composites.This indicates the potential of N-MBP with natural B4C as an effective neutron-shielding material.
Additionally, the simulated results for E-MBP, based on the same B4C ratio as the actual MBP, exhibited a trend similar to the experimental results.This indicates structural uniformity and good agreement between the simulation and experimental findings.

Referee #2
Comments: In this manuscript, a two-dimensional (2D) Ti3C2Tx MXene hybrid film with homogeneously distributed B4C particles was prepared by vacuum filtration and blade coating methods.With high loading of B4C, the hybrid film shows excellent neutron-shielding performance at low thicknesses.The experiment result is interesting and is of some practical interest.In order to further improve the readability and clarity of this work, this reviewer has some comments for authors as listed below.
Reply: We appreciate the reviewer's comment and the request for additional results on the properties of our new composite material.In the revised manuscript, we have included information on the surface and cross-sectional morphologies with element distribution analysis as well as mechanical test results, to further characterize our Ti3C2Tx MXene hybrid films.The provided morphological and elemental analysis, along with mechanical property data, enhance the understanding of the structural characteristics and composition of our painted MBP films, further validating their potential as flexible and robust neutron-shielding materials.

Why B4C was chosen as the neutron shielding filler. Compared with boron and boron nitride,
what are the advantages of B4C?

Reply on Comment (1):
We would like to appreciate to the referee's comment.Boron naturally exists as two stable isotopes, i.e., 10 B and 11 B, at a ratio of 1:4, and its neutron capture ability is mainly attributed to isotope 10 B. Therefore, boron and boron compounds such as boron oxide (B2O3) and boron nitride (BN) are promising candidates for neutron radiation shielding.
However, the surface density of boron, which is directly related to the neutron-shielding capabilities of boron compounds, is prominent in the order of B2O3, h-BN, B4C, and B (Analytica Chimica Acta, 124, 373, (1981)) and (P.Tamayo, (2020) Micro and Nanostructured Composite Materials for Neutron Shielding Applications, Woodhead Publishing).Although boron has a higher neutron-shielding ability than B4C, many impurities in commercially available boron powder produce secondary gamma emissions after irradiation.In this regard, B4C is the most widely used form in neutron-shielding applications.Moreover, the chemical inertness and low price of B4C (B4C: 1.76 $/g, BN: 3.72$/g, and B: 3.85$/g from Sigma Aldrich) render it a conventional neutron-absorbing/shielding material for many nuclear industries, as described in the main text.
The synthesis of pure boron is extremely difficult because of its high reactivity and extreme hardness, which can result in impurities or superficial oxidation.By contrast, boron nitride is not a significant neutron absorber and generally exhibits a smaller macroscopic cross-section than B4C.
In response to the reviewer's questing regarding the advantages of B4C, we have made the necessary corrections in the introduction part and inserted new references of 14-15.

*Revised manuscript
(page 2, line 23) Boron carbide (B4C) is widely used as a neutron-absorbing material because of its high reaction cross-section of 10 B, high melting point of 2,763 K, and usefulness for neutron capture. 12,13By contrast, pure boron and other boron compounds (such as hexagonal boron nitride) exhibit serious purity issues due to its high reactivity and inadequate shielding properties, respectively.Reply on Comment (2): As described in Supplementary Note 3, the interaction between MXene and B4C in the colloidal dispersion is primarily governed by electrostatic repulsion.
owing to the negative surface charge of both materials (Supplementary Figure S10).However, in the case of large B4C (L-B4C) particles, an electrostatic attraction force between MXene and L-B4C was observed, which resulted in the aggregation of MXene flakes on the surface of L-B4C (Supplementary Figure S3).This phenomenon is attributed to the lower presence of boron oxide on the surface of L-B4C, which eliminates the negative surface potential typically associated with B4C.We have now included supplementary data, specifically the surface potential of L-B4C particles, in Supplementary Figure S10a.
Whereas other 2D materials with negative zeta potentials can potentially serve as matrices for B4C, the large flake size of MXene renders it suitable for accommodating B4C within its structure.In our case, the synthesized MXene flakes have an average diameter of approximately 5 m (Supplementary Figure S1), which enhances the mechanical properties of the resulting films.To the best of our knowledge, other 2D materials such as h-BN and MoS2 (which are synthesized via top-down liquid exfoliation method) typically have sub-micron flake sizes and lower production yields, which limit their utility as matrices for B4C and their commercialization potential.

*Revised manuscript
(Supplementary Information, page 5, line 9) To investigate the effect of the particle size on the interaction between Ti3C2Tx and B4C, large B4C (L-B4C) particles were isolated via

Please add the SEM photos of the surface and cross section of the film prepared by blade
coating, in order to illustrate whether there is any difference between the micro-morphology of the film prepared by blade coating and vacuum filtration.

Reply on Comment (3):
The surface and cross-sectional morphologies of the painted films, as shown in Supplementary Figure S17, demonstrate that they possess the same structure as the vacuum-filtrated films.This indicates that the blade coating process did not introduce significant changes to the overall morphology and structure of the films.
To confirm the even distribution of the elements within the painted films, we performed EDS mapping analysis on both the surface and cross-section parts.The EDS mapping results, depicted in Supplementary Figure S18, clearly show that Ti, B, and C were evenly distributed throughout the entire area, suggesting that the n-B4C particles were effectively mixed with the MXene matrix, resulting in a uniform element distribution.

*Revised manuscript
(page 7, line 4) The surface and cross-sectional morphologies displayed in Supplementary Figure S17 showed that the painted films possessed the same structure as vacuum-filtrated ones.
EDS mapping carried out on the surface and the cross-section of painted MBP films further confirmed the even distribution of Ti, B, and C elements throughout the entire area, indicating that the n-B4C was evenly mixed into the MXene matrix (Supplementary Figure S18).

Reply on Comment (4):
We conducted mechanical property tests on the films prepared by blade coating technique and have included the results in the revised manuscript (Supplementary Figure S20).The mechanical properties of our prepared films were comparable to those of the vacuum-filtered film, indicating that the choice of the coating method did not significantly affect the mechanical strength of the MBP film.Additionally, the mechanical strength and failure strain of the hybrid film were primarily determined by the properties of the substrate, highlighting the importance of using robust and flexible substrates to achieve the desired mechanical performance.

*Revised manuscript
(page 7, line 11) Furthermore, it can be observed that the mechanical strength of the nylon membrane painted with an MBP film does not exhibit any remarkable changes (Supplementary Figure S20).The mechanical strength and failure strain of the hybrid film are primarily attributed to the material properties of the substrate, enabling it to withstand uniform stressstrain by introducing robust and flexible substrates.6. Please add serial numbers to the formulas in the manuscript.

Reply on Comment (6):
In the revised version, we have added the serial numbers to the formulas.We thank the reviewer for pointing it out.
7. The English of this article should be further polished.The authors should review the manuscript carefully to improve readability.

Reply on Comment (7):
To further polish the article and improve its readability, we have thoroughly proofed the manuscript for grammar, phrasing, punctuation, and typos.
Response to Reviewers' comments: Thank you very much for your valuable feedback on the revised manuscript.We appreciate your insights and would like to address the concerns raised in your review.Here is our revised manuscript, along with a reply to your comments.For ease of tracking, we are assigning numbers the Reviewer's comments as shown below.Comments are reproduced verbatim in italics, followed by our responses.

Reviewer #1
Remarks to the author: The authors have answered to the comments of the two reviewers and made changes to the manuscript accordingly.The quality of the paper has increased from the modifications and the additional descriptions and data provided.However, major issues either remained from previous version or appearing from the changes, which have great importance to the key information delivered by the article, should be further discussed.Hence, the manuscript is not suitable for publication in current status: Reply: We greatly appreciate the reviewer's insightful comments on our work, as well as his/her suggestions to improve the quality of our paper.We address below the point-by-point reviewer's comments and have made clarifying revisions and expansions to the manuscript.
1. P.2, L23, the added sentences "Boron carbide (B4C) is widely used as a neutron-absorbing material because of its high reaction cross-section of 10  Reply on Comment (1): We agree.We thank the reviewer for pointing it out.We have taken your concerns into consideration and made revisions in the manuscript accordingly.Following the reviewer's first comment, we have changed the sentence discussing the "usefulness for neutron capture" by emphasizing the specific properties of B4C that make it suitable for neutron capture and shielding purposes.
Regarding the second point, we are sorry for any confusion occurred during the last revision process.As the reviewer exactly pointed it out, h-BN possesses desirable properties such as chemical inertness and radiation hardness, which make it a promising material for various shielding applications.In response to the reviewer's questing regarding the advantages of B4C, we have made the necessary corrections in the introduction part as follows.

*Revised manuscript
(page 2, line 24) Boron carbide (B4C) is widely used as a neutron-absorbing material because of its high reaction cross-section of 10 B, high melting point (2,763 K), and low density (2.52 g cm -3 ). 12,13Boron naturally exists as two stable isotopes, i.e., 10 B and 11 B, at a ratio of 1:4, and its neutron capture ability is mainly attributed to isotope 10 B. Therefore, boron and boron compounds such as boron oxide (B2O3) and boron nitride (BN) are promising candidates for neutron radiation shielding.However, the surface density of boron, which is directly related to the neutron-shielding capabilities of boron compounds, is prominent in the order of B2O3, hexagonal BN, B4C, and B. 14,15 Although B has a higher neutron-shielding ability than B4C, many impurities in commercially available boron powder produce secondary gamma emissions after irradiation.In this regard, B4C is the most widely used form in neutron-shielding applications.Therefore, Bem, H., & Ryan, D.E.Choice of boron shield in epithermal neutron activation determinations.Analytica Chimica Acta, 124, 373-380 (1981).Tamayo, P., Thomas, C., Rico, J., Cimentada, A., Setién, J., & Polanco, J. A. Review on neutron-absorbing fillers. in Micro and Nanostructured Composite Materials for Neutron Shielding Applications (ed.Abdulrahman, S. T., Ahmad, Z., & Thomas, S.) 25-52 (Woodhead Publishing, 2020).
With regard to the Ti issue in our MBP samples raised by the reviewer, we first would like to note that the pure Ti3C2Tx film (without B4C and PVA) with small thickness (approximately 40 m) showed limited ability to shield from neutron radiation, given that the I/I0 value (i.e., neutron permeability of the 40-m-thick, pure Ti3C2Tx film) was as high as ≈0.999, as described in the main text in lines of 16-18 on page 8.This indicates that the effect of Ti activation and subsequent emission of gamma rays is negligible in our MBP samples with small thicknesses.We note that our focus was on resolving the inherent challenges of integrating a high concentration of B4C within an ultrathin layer.
For the reviewer's interest, of the Ti isotopes that can be produced by thermal neutrons, only Ti-50 is radioactive.Not only is the probability of its formation (thermal neutron capture cross section) very low [Refs.A & B], but its product, Ti-51, also undergoes a brief period of electron emission (beta decay) with a half-life of 5.76 min, making the activation effect of Ti due to thermal neutrons negligible.Regarding gamma-ray emission of Ti isotopes, the amount of prompt gamma-ray emission that occurs in neutron capture reactions is proportional to their neutron capture cross sections.However, not only is the neutron capture cross section value of Ti very low, that of Ti-49 is also low [Ref. C].Ti-49 is known to emit high-energy gamma rays, and its probability of neutron capture reactions is approximately 1.6% of the entire Ti isotope

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Revised manuscript (page 8 , line 29) In addition, Monte Carlo N-Particle (MCNP) simulations were conducted to examine the transmission probabilities of MBP composite films using a realistic scattering and physics model and to compare the obtained results with experimental measurements.The simulated values of absorption capacity for various B4C contents exhibited a high degree of consistency with the experimental results, thereby demonstrating the accuracy and reliability of the MCNP simulation in assessing the shielding effectiveness of the MBP composite films.(page 9 , line 15) Moreover, to ensure a more precise and unbiased comparison with other the findings of the other studies based on diverse isotopes of B, we extended our simulations to include scenarios in which different types of B4C powder were employed.Specifically, we denoted the simulation results as E-MBP, which represent MBP 40 with 10 B isotope-enriched B4C, and N-MBP, which represents MBP 40 with natural B4C.These composites were characterized by distinct weight fractions of the 10 B isotope compared with those of the 11 B isotope, as shown in the calculations provided in Supplementary Table 52Backis,A.et al.General considerations for effective thermal neutron shielding in detector applications.EPJ Tech.Instrum.9(1), 8 (2022).

Figure 4 .
Figure 4. Neutron-shielding performances of MBP hybrid films.(a) Neutron absorption capacity of MBP hybrid films with different B4C usages.(b) Neutron absorption capacity and calculated macroscopic absorption cross-section of the hybrid films with varying B4C content.The analysis involved a comparison between the experimental and simulated results.(c) Macroscopic cross-sections and (d) neutron absorption capacities vs. the thicknesses of reported boron-based composites.Dashed lines in (c) and (d) represent the fitted data.Detailed neutron-shielding data are presented in TableS4.The bare stainless-steel foil substrate, coated with the hybrid paint with a B4C fraction of 40 wt%, exhibited a negligible absorption capacity.

Figure S15 .
Figure S15.Comparison with a commercially available neutron shielding material.(a) Photograph of MBP 40 and Mirrobor TM .(b) Stress-strain curves of Mirrobor TM .

Figure S22 .
Figure S22.Light weightness and thermal stability of MBP hybrid films.(a) Photograph showing light weightness of MBP hybrid film, which can settle on the top of a dandelion.(b) TGA curves of MBP hybrid films with varying B4C contents and a PVA film.
centrifugation and decantation.The ζ potential of L-B4C exhibited a broader and higher distribution compared with those of AR-B4C and n-B4C.Despite the negative average value of L-B4C, a positive region was clearly observed in the ζ potential, which suggests the presence of an electrostatic attraction force between L-B4C and MXene.This force results in the aggregation of MXene flakes on the surfaces of larger B4C particles, as shown in Supplementary Figure S3.

Figure S17 .
Figure S17.Morphologies of the painted MBP hybrid film on nylon fabric membrane.SEM images of (a) surface and (b and c) cross-section of the blade-coated MBP hybrid film on a nylon membrane.

Figure S18 .
Figure S18.EDS mapping images of painted MBP hybrid films on nylon substrate.Images of (a) surface and (b) cross-section of the blade-coated MBP hybrid film with a B4C weight fraction of 40 wt.% showing the uniform distribution of boron in the film.

Figure S20 .
Figure S20.Stress-strain curves of painted MBP hybrid film with a B4C weight fraction of 40 wt.% on a nylon substrate.

Figure S19 .
Figure S19.Stripping test on the MBP hybrid film painted on a nylon membrane.

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Refs.B & C].As a result, the nuclear industry considers Ti (e.g., the Ti-6Al-V alloy) as a key element, using it in the structural components of nuclear reactors and spent nuclear fuel storage systems [Refs.D & E].Recently, Ti has also been investigated as an alloying material of stainless steel, which is a critical structural material in nuclear reactors.This trend is prominent not only in pressurized water reactors but also in nuclear-fusion and advanced reactors (e.g., high-speed reactors), where Ti alloys or Ti-added metal alloys are being considered as major structural materials[Refs.F&G].

Table S4
Additionally, we appreciate your suggestion to provide more results for the other properties of our MBP composite material based on comparison with these existing materials.
. The bare stainless-steel foil substrate, coated with the hybrid paint with a B4C fraction of 40 wt%, exhibited a negligible absorption capacity.since there are already materials reasonably flexible and robust.Reply on Comment (2):Thank you for your comments.We have carefully investigated the properties of existing flexible neutron-shielding materials, i.e., MirroborTM and SWX-238 Flex      Boron.